This application claims the priority of Japanese Patent Applications No. 212253/1992 filed Jul. 15, 1992 which is incorporated herein by reference.
An audio apparatus, (e.g., stereo, radio, TV or CD player) makes use of an assembly consisting of different speakers (e.g., low frequency speaker (woofer), middle frequency speaker (squawker) and high frequency speaker (tweeter)) to generate sound. A speaker which converts electric power into sound energy comprises an electromechanical converter and a vibration plate which converts the mechanical vibration into sound waves. The vibration plate of a speaker used to be made of paper. Materials used for forming the vibration plate have been developed from paper to metal, e.g. titanium (Ti). The sound velocity on the material is one of several important factors which determines the performance of a vibration plate. The sound velocity is determined by the quotient E/.rho., where E is the Young's modulus and .rho. is the density of the material. The higher the sound velocity is, the more excellent the performance of the vibration plate for high frequency region becomes.
Beryllium (Be) has been known as a material endowed with high E/.rho.. Speakers having beryllium vibration plates have already been produced for improving the response of speakers in the high frequency region. However, beryllium is a poison. The present invention seeks to avoid the use of beryllium in a vibrating plate and thereby account for such considerations as human health and environmental pollution.
Diamond is the material which is favored with the highest E/.rho.. Since diamond has the highest sound velocity of all materials, the diamond vibration plate is considered preferably for the high frequency region. However, currently nobody has succeeded in making a diamond vibration plate for a speaker, although a skilled artisan would know the excellency of diamond for a vibration plate.
Many proposals have been done with respect to diamond speaker vibration plates. Japanese Patent Laying Open No. 61-128700 (128700/'86) defines the relation between the Young's modulus and the density of materials. Japanese Patent Laying Open No. 1-100277 (100277/'89) proposes a speaker vibration plate made from hard, carbon film. The proposed vibration plate are not diamond but a hard carbon film having a high E/.rho..
Japanese Patent Laying Open No. 62-152299 disclosed a method for making a diamond-like carbon film as a vibration plate, wherein the vibration plate is produced by depositing a diamond-like film on a substrate by the ion plating method and by eliminating the substrate by dissolving it. Japanese Patent Publication No. 55-33237 (33237/'80) provides for the manufacture of a quasi-diamond carbon film as a speaker vibration plate by the ion beam evaporation method. Japanese Patent Publication No. 4-23480 (23480/'92) discloses a method of making a vibration plate of a speaker, wherein the vibration plate is produced by depositing a diamond film on a dome-shaped silicon substrate by the CVD method and by eliminating the silicon substrate by dissolving with some etchant.
Diamond is sure to be the most preferable material for a vibration plate from the standpoint of large E/.rho. or providing large sound velocity. There have been many proposals for various diamond vibration plates for speakers.
Every previously proposed diamond vibration plate lacks sufficient consideration to a singular shaped vibration plate. Hence, a speaker vibration plate is not a flat plate but a dome-shaped plate with a half-spherical part (A) and an external, circular flange (C) as shown in FIG. 1 or FIG. 11. The central spherical part and the annular flange have different roles and different inner stresses, and suffer different external forces. Especially, the periphery of the flange is apt to receive strong external stress. Every prior vibration plate provided in the art discloses a central half-sphere and a circular flange made from the same material. The uniformity of material is a common feature of almost all conventional vibration plates. The central spherical part which is not fixed to anything vibrates in high frequency for converting mechanical vibration into sound vibration. Thus, the central part requires a high sound velocity for improving the high frequency performance. By contrast, the periphery of the flange is fixed to something such as a peripheral metal part of a speaker equipped in a radio headphones or TV set. Since the flange which supports the central part is fixed to something, it cannot always deform freely. An external force certainly acts on the flange, because the flange contacts with some external parts. Larger inner stress remains in the flange rather than in the half-sphere. Therefore, high toughness is also important for the vibration plate of a speaker especially for the circular flange.
The conventional materials for vibration plates (e.g., paper, titanium (Ti) or beryllium (Be)) are less attractive than diamond, because they have lower Young's modulus or lower rigidity than diamond. However, the conventional materials enjoy high toughness. The vibration plates made from paper, titanium or beryllium are unlikely to break or split in spite of the repetitions of vibrations or external shocks. These materials have been established as materials for vibration plates. But diamond is not considered a practically-established material for vibration plates. Indeed, E/.rho. of diamond is very high, but high E means high rigidity. The highness of rigidity is apt to lower the toughness in many cases. In the case of diamond vibration plates, the high rigidity should induce breaks or splits of the plates. Weak resistance of diamond against repetitions of vibrations or external shocks has hindered diamond from being used as a material of vibration plates of speakers. Diamond vibration plates have never been practically used in audio apparatuses in spite of many proposals. The rigidity of diamond also invites a difficulty of production. When a diamond film is deposited on a substrate by a CVD method and the substrate is eliminated by acid, the diamond film is apt to break in the solution, because the diamond film misses the substrate as a supporter and inner large stress acts on splitting the film. Thus, the production of diamond vibration plates has not been put to practical use.
One purpose of this invention is to provide a tough diamond vibration plate which is immune from breaks or splits. Another purpose of this invention is to provide a method for producing a diamond vibration plate with high yield. Still another purpose of this invention is to provide a diamond vibration plate which is cheaper than the prior diamond vibration plates.